Electromigration Performance of WLCSP Solder Joints

Abstract: Wafer Level Chip Scale Package (WLCSP) assemblies were tested under high temperature and high current conditions. Electromigration damage was observed along with accelerated diffusion and intermetallic compound growth at the solder / Under Bump Metallization (UBM) interface. Final electrical failure typically occurred due to a void created in the redistribution line (RDL) near the UBM. The failure rate increased with higher temperature, higher current density, and reduced RDL trace width. Both Ni UBM pads and Cu pillar structures had superior performance over Cu UBM pads. A failure model based on Black’s equation was developed from the experimental data and other published data. The model was then used to develop recommended guidelines for accelerated testing and qualification testing based on representative field use conditions.